1. Field of the Invention
The present invention relates to cutting tools for surface shaping machine tools and, more particularly, to a cutting tool assembly with a tool holder and an interchangeable tool insert.
2. Description of the Prior Art
In connection with machine tools for the surface shaping of workpieces with cutting tools, where these tools have a certain minimal size and, as a rule, have carbide-tipped metal or ceramic cutting edges, efforts are being made to provide cutting tool assemblies which can be separated at a given separation point into a tool insert which carries the cutting edge and a tool holder by means of which the tool assembly is attachable to the machine tool. The two components of these tool assemblies are attachable to each other by means of a coupling device. It thereby becomes possible to replace highly stressed cutting edges with new ones, or to change from a given cutting edge shape to a different shape more quickly and easily, as compared to compact tools.
The coupling devices of these separable tool assemblies feature cooperating positioning and abutting faces on the tool insert and on the tool holder. They include a clamping mechanism with a traction rod which is movable in relation to the tool holder and operably connected to a traction rod drive, the traction rod being attachable to the tool insert, in order to clamp the latter against the tool holder.
A known example of a separable turning tool assembly features prismatic positioning and abutting faces on the separation point between the tool holder and the tool insert. The traction rod has a cylindrical traction head of a keyhole-shaped contour which engages a recess in the tool insert with a matching keyhole-shaped contour, the recess being open on its short side. This prior art example allows for the tool insert to be connected to the tool holder in only one particular orientation. The coupling and separation of the tool insert and tool holder takes place on a movement path which is perpendicular to the longitudinal axis of the tool shaft. As a result, the range of application of such a tool is limited, especially when, in the course of a machining operation on a workpiece, the main cutting forces fluctuate in size and direction.
Another known separable tool assembly has partially cylindrical positioning and abutting faces at its point of separation. These faces form a ring-cylindrical collar on the tool insert and a stepped cylindrical extension on the tool holder which fits into the collar of the tool insert. These faces thereby transmit the axial and radial cutting forces. Tangential forces are transmitted by means of a radially oriented key-and-groove connection in the area of the planar end face. The coupling device also includes several clamping bolts which are arranged for movement inside outwardly and rearwardly inclined bores and cooperate with an annular wedge on the extremity of the traction rod which forces them outwardly into contact with a taper on the inside of the collar of the tool insert. The inclined orientation of the clamping bolts produces a partially axial and partially radial clamping action. The radial clamping forces act on the interface between the cylindrical faces of the collar on the tool insert and on the extension of the tool holder, so that the wall thickness of the collar cannot be reduced at will. It follows that the radial distance of the clamping faces of the clamping bolts from the longitudinal axis of the tool assembly is comparatively short, so that this coupling device is capable of withstanding only comparatively small bending forces.
Still another known example of a separable tool assembly suggests the arrangement of the positioning and abutting faces on the separation point in the form of planar radial serrations (so-called Hirth serrations). The coupling device includes a clamping shaft which is attached to the tool insert and has coupling faces which form part of a truncated cone. On the inside of the tool holder is arranged a clamping collet with jaws that are guided in part on conical and in part on cylindrical guide faces and have interior contact faces which cooperate with the conical faces of the clamping shaft. A closing movement of the clamping collet, produced by the traction rod, causes the collet to move against the clamping shaft so as to clamp the two planar radial serrations against each other, while simultaneously clamping the clamping shaft in the radial sense. The planar radial serrations transmit axial, radial and tangential forces from the tool insert to the tool holder. The radial orientation of the teeth of the planar radial serrations limits the transmission of radial forces to those teeth which are oriented transversely to the direction of force. Due to the fact that the flanks of those theeth which are not oriented parallel to the direction of force are inclined in relation to the a radial plane, the transmission of a radial force or of a transverse force creates a not inconsiderable axial force component. The transmission of tangential forces likewise produces a considerable axial force component. Because the cutting edge on the tool insert is normally arranged out of center, often at a considerable distance from the tool axis, the axial components of these forces add up to considerable force levels on one side. An axial force component tends to separate the planar radial serrations. This axial force component, therefore, must additionally be absorbed by the clamping device.